U.S. patent application number 11/991955 was filed with the patent office on 2009-10-22 for vinylidene fluoride resin hollow fiber porous membrane and method for production thereof.
Invention is credited to Masayuki Hino, Toshiya Mizuno, Kenichi Suzuki, Yasuhiro Tada, Takeo Takahashi.
Application Number | 20090261034 11/991955 |
Document ID | / |
Family ID | 37864923 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090261034 |
Kind Code |
A1 |
Takahashi; Takeo ; et
al. |
October 22, 2009 |
Vinylidene Fluoride Resin Hollow Fiber Porous Membrane and Method
for Production Thereof
Abstract
A hollow-fiber porous membrane comprising a hollow-fiber form of
vinylidene fluoride resin and satisfying the following properties
(A) and (B), is provided as a hollow-fiber porous membrane of
vinylidene fluoride resin having an average pore size and a further
uniform pore size distribution (A) and a large water permeation
rate regardless of good efficiency of blocking minute particles
(bacteria) (B), as represented by: (A) a ratio Pmax (1 m)/Pm of at
most 4.0 between a maximum pore size Pmax (1 m) measured at a test
length of 1 m according to the bubble point method and an average
pore size Pm of 0.05-0.20 .mu.m measured according to the half dry
method; and (B) a ratio F (L=200 mm, v=70%)/PM.sup.2 of at least
3000 (m/day.mu.m.sup.2), wherein the ratio F (L=200 mm,
v=70%)/Pm.sup.2 denotes a ratio between F (L=200 mm, v=70%) which
is a value normalized to a porosity v=70% of a water permeation
rate F (100 kPa, L=200 mm) measured at a test length L=200 mm under
the conditions of a pressure difference of 100 kPa and a water
temperature of 25.degree. C. and a square Pm.sup.2 of an average
pore size Pm. The hollow-fiber porous membrane is produced by a
process, comprising: subjecting a stretched hollow-fiber porous
membrane of vinylidene fluoride resin to at least two stages of
relaxation treatment in a non-wetting environment.
Inventors: |
Takahashi; Takeo;
(Ibaraki-Ken, JP) ; Tada; Yasuhiro; (Ibaraki-Ken,
JP) ; Suzuki; Kenichi; (Tokyo, JP) ; Hino;
Masayuki; (Ibaraki-Ken, JP) ; Mizuno; Toshiya;
(Ibaraki-Ken, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
37864923 |
Appl. No.: |
11/991955 |
Filed: |
September 12, 2006 |
PCT Filed: |
September 12, 2006 |
PCT NO: |
PCT/JP2006/318028 |
371 Date: |
March 13, 2008 |
Current U.S.
Class: |
210/500.23 ;
264/342RE |
Current CPC
Class: |
B01D 69/02 20130101;
B01D 2323/20 20130101; B01D 2323/12 20130101; B01D 71/34 20130101;
B01D 2313/24 20130101; B01D 63/024 20130101; D01F 6/12 20130101;
D01D 5/247 20130101; C02F 1/444 20130101; B01D 67/0027 20130101;
B01D 69/08 20130101; B01D 67/0086 20130101; B01D 69/087
20130101 |
Class at
Publication: |
210/500.23 ;
264/342.RE |
International
Class: |
B01D 71/34 20060101
B01D071/34; B01D 69/08 20060101 B01D069/08 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 2005 |
JP |
2005-266279 |
Claims
1. A hollow-fiber porous membrane, comprising a hollow fiber-form
porous membrane of vinylidene fluoride resin and satisfying the
following properties (A) and (B): (A) a ratio Pmax (1 m)/Pm of at
most 4.0 between a maximum pore size Pmax (1 m) measured at a test
length of 1 m according to the bubble point method and an average
pore size Pm of 0.05-0.20 .mu.m measured according to the half dry
method; and (B) a ratio F (L=200 mm, v=70%)/Pm.sup.2 of at least
3000 (m/day.mu.m.sup.2), wherein the ratio F (L=200 mm,
v=70%)/Pm.sup.2 denotes a ratio between F (L=200 mm, v=70%) which
is a value normalized to a porosity v=70% of a water permeation
rate F (100 kPa, L=200 mm) measured at a test length L=200 mm under
the conditions of a pressure difference of 100 kPa and a water
temperature of 25.degree. C. and a square Pm.sup.2 of an average
pore size Pm.
2. A hollow-fiber porous membrane according to claim 1, wherein the
ratio Pmax (1 m)/Pm is at most 3.0.
3. A hollow-fiber porous membrane according to claim 1, wherein the
ratio F (L=200 mm, v=70%)/Pm.sup.2 is at least 3500
(m/day.mu.m.sup.2).
4. A process for producing a hollow-fiber porous membrane of
vinylidene fluoride resin, comprising: subjecting a stretched
hollow-fiber porous membrane of vinylidene fluoride resin to at
least two stages of relaxation treatment in a non-wetting
environment.
5. A production process according to claim 4, wherein the
non-wetting environment for the two stages of relaxation treatment
comprises water and/or air.
6. A production process according to claim 5, wherein the water
and/or air comprises water at 50-100.degree. C. and/or air or steam
at 80-160.degree. C.
7. A production process according to claim 5, wherein the
non-wetting environment for the two stages of relaxation treatment
successively comprise water, and air or steam.
8. A production process according to claim 4, wherein the two
stages of relaxation are performed at relaxation percentages of
2-20% for each stage and totally 4-30%.
9. A production process according to claim 4, wherein the stretched
hollow-fiber porous membrane of vinylidene fluoride resin has been
formed by melt-extruding into a hollow-fiber film a starting
mixture comprising the vinylidene fluoride resin and at least a
plasticizer for vinylidene fluoride resin, followed by cooling,
extraction of the plasticizer and stretching.
10. A production process according to claim 9, wherein the starting
mixture further comprises a good solvent for vinylidene fluoride
resin.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hollow-fiber porous
membrane (hollow fiber-form porous membrane) of vinylidene fluoride
resin excellent in water (filtration) treatment performances, and a
process for production thereof.
BACKGROUND ART
[0002] Vinylidene fluoride resin is excellent in chemical
resistance, heat resistance and mechanical strength and, therefore,
has been studied with respect to application thereof to porous
membranes for separation. In the case of use for water (filtration)
treatment, particularly for production of potable water or sewage
treatment, a hollow fiber-form porous membrane is frequently used
because it can easily provide a large membrane area per unit volume
of filtration apparatus, and many proposals have been made
including processes for production thereof (e.g., Patent documents
1-3 listed below).
[0003] Also, the present inventors, et al., have found that a
process of melt-extruding a vinylidene fluoride resin having a
specific molecular weight characteristic together with a
plasticizer and a good solvent for the vinylidene fluoride resin
into a hollow fiber-form and then removing the plasticizer by
extraction to render the hollow fiber porous is effective for
formation of a porous membrane of vinylidene fluoride resin having
minute pores of appropriate size and distribution and also
excellent in mechanical strength, and have made a series of
proposals (Patent document 4 listed below, etc.). However, a strong
demand exists for further improvements of overall performances
including filtration performances and mechanical performances of
the hollow-fiber porous membrane necessary for use as a filtration
membrane. It is particularly desired to have pores having
appropriate sizes for removing particles to be removed and a
further uniform distribution, in addition to a large water
permeation rate (filtration performance).
[0004] For example, the present inventors, et al. have found that
the relaxation treatment of a stretched porous membrane of
vinylidene fluoride resin under a wetting condition in a liquid
which wets the porous membrane of vinylidene fluoride resin causes
a great effect in increasing the water permeation rate of the
resultant porous membrane of vinylidene fluoride resin and have
proposed a process for producing a hollow-fiber porous membrane of
vinylidene fluoride resin (Patent document 5). However, according
to a result of the present inventors' study, it has been found that
the hollow-fiber porous membrane of vinylidene fluoride resin
obtained through the process, while it actually has a remarkably
increased water permeation rate, is liable to show a pore size
distribution including a main peak and also another small sub-peak
on a larger pore size side of the main peak and is further liable
to have pinholes leading to a lowering in performance of removing
minute particles or bacteria during water (filtration)
treatment.
[0005] Patent document 1: JP-A 63-296939
[0006] Patent document 2: JP-A 63-296940
[0007] Patent document 3: JP-A 3-215535
[0008] Patent document 4: WO2004/081109A
[0009] Patent document 5: JP-A 2006-63095
DISCLOSURE OF INVENTION
[0010] The present invention aims at providing a hollow-fiber
porous membrane of vinylidene fluoride resin having pores of an
appropriate size (average pore size) and a further uniform pore
size distribution in addition to a large water permeation rate, and
a process for production thereof.
[0011] A further object of the present invention is to provide a
hollow-fiber porous membrane of vinylidene fluoride resin
exhibiting a large water permeability in spite of a relatively
small average pore size, and a process for production thereof.
[0012] The porous membrane of vinylidene fluoride resin according
to the present invention has been developed so as to accomplish the
above-mentioned objects and comprises a hollow fiber-form porous
membrane of vinylidene fluoride resin and satisfying the following
properties (A) and (B); i.e., (A) a ratio Pmax (1 m)/Pm of at most
4.0 between a maximum pore size Pmax (1 m) measured at a test
length of 1 m according to the bubble point method and an average
pore size Pm of 0.05-0.20 .mu.m measured according to the half dry
method; and (B) a ratio F (L=200 mm, v=70%)/Pm.sup.2 of at least
3000 (m/day.mu.m.sup.2), wherein the ratio F (L=200 mm,
v=70%)/Pm.sup.2 denotes a ratio between F (L=200 mm, v=70%) which
is a value normalized to a porosity v=70% of a water permeation
rate F (100 kPa, L=200 mm) measured at a test length L=200 mm under
the conditions of a pressure difference of 100 kPa and a water
temperature of 25.degree. C. and a square Pm.sup.2 of an average
pore size Pm.
[0013] Herein, the above-mentioned property (A) of the hollow-fiber
porous membrane of vinylidene fluoride resin according to the
present invention represents a small average pore size and also a
uniform pore size distribution, and the property (B) represents a
large normalized water permeation rate F (L=200 mm, v=70%)
regardless of the small average pore size Pm. Particularly, the
property (A) represents the absence of pinholes which are believed
to be associated with the liability of a sub-peak on a larger pore
size side in the hollow-fiber membrane obtained through the
above-mentioned process of Patent document 5, and is particularly
important in evaluating the performances of the hollow-fiber
membrane of the present invention. At a test length of 10-20 mm
generally adopted in measurement of maximum pore size Pmax
according to the bubble point method, the measured Pmax values
fluctuate sample to sample, but at a test length L increased up to
1 m, the rate of capturing pinholes is increased so that the
presence or absence of pinholes in a sample of the objective
hollow-fiber membrane can be surely judged. (Refer to Table 1
appearing hereinafter indicating the measurement of Pmax values for
the hollow-fiber membranes produced according to Example 1 and
Comparative Example 1 at an increased number of samples thereof).
Further the test length of 1 m is also useful in evaluating the
suitableness of a hollow-fiber membrane of vinylidene fluoride
resin as a long hollow-fiber membrane because of a less increase in
flow resistance through the hollow-fiber at a larger length.
[0014] As a result of the present inventors' study, it has been
found effective to adopt a multi-stage relaxation treatment in a
non-wetting environment instead of the relaxation in a wetting
liquid adopted in the above-mentioned process of Patent document 5
for the purpose of producing the hollow-fiber porous membrane of
vinylidene fluoride resin according to the present invention
characterized by the above-mentioned properties (A) and (B). As a
result, it becomes possible to obviate the generation of a sub-peak
in particle size distribution leading to the formation of pinholes
due to the relaxation in a wetting liquid in the process of Patent
document 5. Thus, according to the present invention, there is also
provided a process for producing a hollow-fiber porous membrane of
vinylidene fluoride resin, comprising subjecting a stretched
hollow-fiber porous membrane of vinylidene fluoride resin to at
least two stages of relaxation treatment in a non-wetting
environment.
[0015] At the time when the process of Patent document 5 was
developed; the present inventors, et al., believed that the effect
of increase in water permeation rate due to the relaxation in a
wetting liquid was largely attributable to the effect of further
extraction of residual plasticizer exposed to pore walls formed
anew due to the stretching, but the present inventors now believe
that the increase in water permeation rate is principally
attributable to the transformation due to relaxation of an
elongated oval-shape of pores formed immediately after the
stretching to approach a true circle (giving a maximum aperture
area at an identical circumferential length), in view of the effect
of increase in water permeation rate attained by relaxation in
non-wetting environment (of, typically, water or air) according to
the present invention. The reason for a larger effect attained by
the multi-stage (typically, two-stage) relaxation than a
single-stage relaxation (as understood from a comparison between
Example 3 and Comparative Example 5 appearing hereinafter) is not
necessarily clear, but some assumptions may be possible such that
the history of a first-stage relaxation generally performed under
heating may advantageously affect the transformation to a true
circle in a second-stage relaxation, and geometrically different
relaxation mechanisms (e.g., two-dimensional one in addition to a
single dimensional one) may be used in the two-stage relaxation
process in view of the fact that the multi-stage relaxation
resulted in a larger relaxation percentage than the single-stage
relaxation for hollow-fibers of an identical stretch ratio.
Further, such a larger relaxation percentage attained by the
multi-stage relaxation can result in a larger increase in water
permeation rate by that much.
[0016] Incidentally, Patent documents 2 and 3 also disclose a
process for producing a hollow-fiber membrane of vinylidene
fluoride resin including a relaxation treatment of a hollow-fiber
membrane after stretching. However, the relaxation disclosed
therein is a single-stage treatment and, while the combined effects
of stretching-relaxation, a lowering in tensile modulus and an
increase in water permeation rate were conceived, it was not
clarified as to how the relaxation treatment after the stretching
should be performed, particularly regarding a difference in
increase of water permeation rate due to a difference in the
relaxation treatment alone.
BRIEF DESCRIPTION OF THE DRAWING
[0017] FIG. 1 is a schematic illustration of a water
permeability-metering apparatus for evaluating water-treating
performances of hollow-fiber porous membranes obtained in Examples
and Comparative Examples.
BEST MODE FOR PRACTICING THE INVENTION
[0018] Hereinbelow, the hollow-fiber porous membrane of vinylidene
fluoride resin of the present invention will be described in the
order of the production process of the present invention that is a
preferred process for production thereof.
[0019] (Vinylidene Fluoride Resin)
[0020] In the present invention, a vinylidene fluoride resin having
a weight-average molecular weight of
2.times.10.sup.5-6.times.10.sup.5 is preferably used as a principal
membrane-forming material. If Mw is below 2.times.10.sup.5, the
mechanical strength of the resultant porous membrane becomes small.
On the other hand, if Mw exceeds 6.times.10.sup.5, the texture of
phase separation between the vinylidene fluoride resin and the
plasticizer tends to become excessively fine to result in a porous
membrane exhibiting a lower water permeation rate when used as a
microfiltration membrane.
[0021] The vinylidene fluoride resin used in the present invention
may be homopolymer of vinylidene fluoride, i.e., polyvinylidene
fluoride, or a copolymer of vinylidene fluoride together with a
monomer copolymerizable with vinylidene fluoride, or a mixture of
these. Examples of the monomer copolymerizable with vinylidene
fluoride may include: tetrafluoroethylene, hexafluoropropylene,
trifluoroethylene, chlorotrifluoroethylene and vinylidene fluoride,
which may be used singly or in two or more species. The vinylidene
fluoride resin may preferably comprise at least 70 mol % of
vinylidene fluoride as the constituent unit. Among these, it is
preferred to use homopolymer consisting of 100 mol % of vinylidene
fluoride in view of its high mechanical strength.
[0022] A vinylidene fluoride resin of a relatively high vinylidene
fluoride content as described above may preferably be obtained by
emulsion polymerization or suspension polymerization, particularly
preferably by suspension polymerization.
[0023] The vinylidene fluoride resin forming the hollow-fiber
porous membrane of the present invention is preferably one
characterized by good crystallinity, i.e., a crystalline property
of suppressing growth of spherulites but promoting the formation of
network texture in the course of cooling, as represented by a
difference Tm2-Tc of at most 32.degree. C., preferably at most
30.degree. C., between an inherent melting point Tm2 (.degree. C.)
and a crystallization temperature Tc (.degree. C.) of the resin as
determined by DSC measurement in addition to the above-mentioned
relatively large weight-average molecular weight of
2.times.10.sup.5-6.times.10.sup.5.
[0024] Herein, the inherent melting point Tm2 (.degree. C.) of
resin should be distinguished from a melting point Tm1 (.degree.
C.) determined by subjecting a procured sample resin or a resin
constituting a porous membrane as it is to a temperature-increase
process according to DSC. More specifically, a vinylidene fluoride
resin procured generally exhibits a melting point Tm1 (.degree. C.)
different from an inherent melting point Tm2 (.degree. C.) of the
resin, due to thermal and mechanical history thereof received in
the course of its production or heat-forming process, etc. The
melting point Tm2 (.degree. C.) of vinylidene fluoride resin
defining the present invention is defined as a melting point (a
peak temperature of heat absorption according to crystal melting)
observed in the course of DSC re-heating after once subjecting a
procured sample resin to a prescribed temperature increase and
decrease cycle in order to remove the thermal and mechanical
history thereof, and details of the measurement method will be
described prior to the description of Examples appearing
hereinafter.
[0025] The condition of Tm2-Tc.ltoreq.32.degree. C. representing
the crystallinity of vinylidene fluoride resin forming the porous
membrane of the present invention may possibly be accomplished,
e.g., by a lowering in Tm2 according to copolymerization, but in
this case, the resultant hollow fiber porous membrane is liable to
have a lower chemical resistance in some cases. Accordingly, in a
preferred embodiment of the present invention, there is used a
vinylidene fluoride resin mixture formed by blending 70-98 wt. % of
a vinylidene fluoride resin having a weight-average molecular
weight molecular weight of 1.5.times.10.sup.5-6.times.10.sup.5 as a
matrix (or principal) resin and 2-30 wt. % of a high-molecular
weight vinylidene fluoride resin having an Mw that is at least 1.8
times, preferably at least 2 times, that of the former and at most
1.2.times.10.sup.6, for crystallinity modification. According to
such a method, it is possible to significantly increase the
crystallization temperature Tc without changing the crystal melting
point of the matrix resin alone (represented by Tm2 in a range of
preferably 170-180.degree. C.). More specifically, by increasing
Tc, in the case of a preferential cooling from an outer surface of
a hollow-fiber film formed by melt extrusion, it becomes possible
to accelerate the solidification of the vinylidene fluoride resin
at a region from an inner portion of the film toward an inner
surface, where the cooling is retarded compared with the film
surface, thereby suppressing the growth of spherulites. Tc is
preferably at least 143.degree. C.
[0026] If Mw of the high-molecular weight vinylidene fluoride resin
is below 1.8 times Mw of the matrix resin, it becomes difficult to
sufficiently suppress the growth of spherulites. On the other hand,
above 1.2.times.10.sup.6, the dispersion thereof in the matrix
resin becomes difficult.
[0027] Further, if the addition amount of the high-molecular weight
vinylidene fluoride resin is below 2 wt. %, the effect of
suppressing spherulite texture formation is liable to be
insufficient, and in excess of 30 wt. %, the texture of phase
separation between the vinylidene fluoride resin and the
plasticizer is liable to become excessively fine, thus lowering the
water permeation rate of the resultant membrane.
[0028] According to a preferred embodiment of the present
invention, a plasticizer and a good solvent for vinylidene fluoride
resin are added to the above-mentioned vinylidene fluoride resin to
form a starting composition for formation of the membrane.
[0029] (Plasticizer)
[0030] The hollow-fiber porous membrane of the present invention is
principally formed of the above-mentioned vinylidene fluoride
resin, but for the production thereof, it is preferred to use at
least a plasticizer for vinylidene fluoride resin as a pore-forming
agent in addition to the vinylidene fluoride resin. As the
plasticizer, aliphatic polyesters of a dibasic acid and a glycol
may generally be used. Examples thereof may include: adipic
acid-based polyesters of, e.g., the adipic acid-propylene glycol
type, and the adipic acid-1, 3-butylene glycol type; sebacic
acid-based polyesters of, e.g., the sebacic acid-propylene glycol
type; and azelaic acid-based polyesters of, e.g., the azelaic
acid-propylene glycol type, and azelaic acid-1, 3-butylene glycol
type.
[0031] (Good Solvent)
[0032] Further, in order to form the hollow-fiber membrane of the
present invention through melt extrusion at a relatively low
viscosity, it is preferred to use a good solvent for vinylidene
fluoride resin in addition to the above-mentioned plasticizer. As
the good solvent, those capable of dissolving vinylidene fluoride
resin in a temperature range of 20-250.degree. C. may be used.
Examples thereof may include: N-methyl-pyrrolidone,
dimethylformamide, dimethylacetamide, dimethyl sulfoxide, methyl
ethyl ketone, acetone, tetrehydrofuran, dioxane, ethyl acetate,
propylene carbonate, cyclohexane, methyl isobutyl ketone, dimethyl
phthalate, and solvent mixtures of these. N-methylpyrrolidone (NMP)
is particularly preferred in view of its stability at high
temperatures.
[0033] (Composition)
[0034] The starting composition for formation of the hollow-fiber
membrane may preferably be obtained by mixing 100 wt. parts of the
vinylidene fluoride resin with the plasticizer and the good solvent
for vinylidene fluoride resin in a total amount of 100-300 wt.
parts, more preferably 140-220 wt. parts, including 12.5-35 wt. %
thereof, more preferably 15.0-32.5 wt. % thereof, of the good
solvent.
[0035] If the total amount of the plasticizer and the good solvent
is too small, the viscosity of the composition at the time of
melt-extrusion becomes excessively high. If the total amount is too
large, the viscosity is excessively lowered. In both cases, it
becomes difficult to obtain a porous hollow-fiber having a
uniformly and appropriately high porosity, and thus filtration
performance (water permeability). Further, if the proportion of the
good solvent in the total amount of the both components is below
12.5 wt. %, it becomes difficult to attain the effect of uniform
pore size which is a characteristic of the present invention. On
the other hand, if the proportion of the good solvent exceeds 35
wt. %, the crystallization of the resin in the cooling bath becomes
insufficient, thus being liable to cause the collapse of the
hollow-fiber, so that the formation of the hollow-fiber per se
becomes difficult.
[0036] (Mixing and Melt-Extrusion)
[0037] The melt-extrusion composition may be extruded into a hollow
fiber film by extrusion through an annular nozzle at a temperature
of 140-270.degree. C., preferably 150-200.degree. C. Accordingly,
the manners of mixing and melting of the vinylidene fluoride resin,
plasticizer and good solvent are arbitrary as far as a uniform
mixture in the above-mentioned temperature range can be obtained
consequently. According to a preferred embodiment for obtaining
such a composition, a twin-screw kneading extruder is used, and the
vinylidene fluoride resin (preferably in a mixture of a principal
resin and a crystallinity-modifier resin) is supplied from an
upstream side of the extruder and a mixture of the plasticizer and
the good solvent is supplied at a downstream position to be formed
into a uniform mixture until they pass through the extruder and are
discharged. The twin-screw extruder may be provided with a
plurality of blocks capable of independent temperature control
along its longitudinal axis so as to allow appropriate temperature
control at respective positions depending on the contents of the
materials passing therethrough.
[0038] (Cooling)
[0039] Then, the melt-extruded hollow fiber film is cooled
preferentially from an outside thereof and solidified by
introducing it into a cooling liquid bath. In this instance, if the
hollow-fiber film is cooled while an inert gas, such as air or
nitrogen, is injected into the hollow part thereof, a hollow-fiber
film having an enlarged diameter can be obtained. This is
advantageous for obtaining a hollow-fiber porous membrane which is
less liable to cause a lowering in water permeation rate per unit
area of the membrane even at an increased length of the
hollow-fiber membrane (WO2005/03700A). As the cooling liquid, a
liquid which is inert (i.e., showing non-solvency and
non-reactivity) with respect to vinylidene fluoride resin, is
generally used, and preferably water is used. In some case, a good
solvent for vinylidene fluoride resin (similar to those used in the
above-mentioned melt-extrusion composition) which is miscible with
the cooling liquid (preferably, NMP miscible with water) can be
mixed at a proportion of 30-90 wt. %, preferably 40-80 wt. %, of
the cooling liquid, so as to enlarge the pore size at the outer
surface of the resultant hollow-fiber porous membrane, whereby it
becomes possible to obtain a hollow-fiber porous membrane having a
layer of minimum pore size inside the membrane, which is
advantageous for regeneration by air scrubbing (WO2006/087963A).
The temperature of the cooling medium may be selected from a fairly
wide temperature range of 0-120.degree. C., but may preferably be
in a range of 5-100.degree. C., particularly preferably
5-80.degree. C.
[0040] (Extraction)
[0041] The cooled and solidified hollow fiber film is then
introduced into an extraction liquid bath to remove the plasticizer
and the good solvent therefrom, thereby forming a hollow fiber
membrane. The extraction liquid is not particularly restricted
provided that it does not dissolve the vinylidene fluoride resin
while dissolving the plasticizer and the good solvent. Suitable
examples thereof may include: polar solvents having a boiling point
on the order of 30-100.degree. C., inclusive of alcohols, such as
methanol and isopropyl alcohol, and chlorinated hydrocarbons, such
as dichloromethane and 1,1,1-trichloroethane.
[0042] (Stretching)
[0043] The hollow-fiber film or membrane after the extraction may
preferably be subjected to stretching in order to increase the
porosity and pore size and improve the strength-elongation
characteristic thereof. The stretching may preferably be effected
as a uniaxial stretching in the longitudinal direction of the
hollow-fiber membrane by means of, e.g., a pair of rollers rotating
at different circumferential speeds. This is because it has been
found that a microscopic texture including a stretched fibril
portion and a non-stretched node portion appearing alternately in
the stretched direction is preferred for the hollow-fiber porous
membrane of vinylidene fluoride resin of the present invention to
exhibit a harmony of porosity and strength-elongation
characteristic thereof. The stretching ratio may suitably be on the
order of 1.2-4.0 times, particularly 1.4-3.0 times. If the
stretching ratio is too low, it becomes impossible to attain a
large relaxation percentage either so that it becomes difficult to
attain the effect of increasing the water permeation rate. Further,
at an excessively large stretching ratio, the hollow-fiber membrane
can be broken at a high liability. The stretching temperature may
preferably be 25-90.degree. C., particularly 45-80.degree. C. At
too low a stretching temperature, the stretching becomes
nonuniform, thus being liable to cause the breakage of the
hollow-fiber membrane. On the other hand, at an excessively high
temperature, enlargement of pore sizes cannot be attained even at
an increased stretching ratio, so that it becomes difficult to
attain an increased water permeation rate even after the
relaxation. It is preferred to heat-treat the hollow-fiber film or
membrane for 1 sec.-18000 sec., preferably 3 sec.-3600 sec., in a
temperature range of 80-160.degree. C., preferably 100-140.degree.
C., to increase the crystallinity in advance of the stretching for
the purpose of improving the stretchability.
[0044] According to the process of the present invention, the
hollow-fiber porous membrane of vinylidene fluoride resin obtained
through the above-mentioned steps is subjected to at least two
stages of relaxation treatment in a non-wetting environment (or
medium). The non-wetting environment may be formed of non-wetting
liquids having a surface tension (JIS K6768) larger than a wet
tension of vinylidene fluoride resin, typically water, or almost
all gases including air as a representative, particularly gases
which do not condense around room temperature and vapors of the
above-mentioned non-wetting liquids. In order to attain a large
relaxation effect by a treatment at a relatively low temperature
for a short period, it is preferred to effect a treatment with a
non-wetting liquid having a large heat capacity and a large heat
conductivity (i.e., a wet heat treatment), but it is also
preferably adopted to apply a treatment within a heated gas (or
vapor) at a sufficiently high relaxation temperature (i.e., a dry
heat treatment). It is preferred to apply a wet heat treatment
within water at 25-100.degree. C., particularly 50-100.degree. C.,
and/or a dry heat treatment with air (or steam) at 80-160.degree.
C. It is particularly preferred to apply a two-stage relaxation
treatment including a first stage of wet heat treatment in water
and a second stage of wet heat treatment in water or dry heat
treatment in air (or steam).
[0045] The relaxation in each stage may be effected by passing a
hollow-fiber porous membrane stretched in advance through the
above-mentioned non-wetting, preferably heated environment disposed
between an upstream roller and a downstream roller rotating at
successively decreasing circumferential speeds. The relaxation
percentage determined by (1-(the downstream roller circumferential
speed/the upstream roller circumferential speed)).times.100 (%) may
preferably be 2-20% for each stage and ca. 4-30% as a total
relaxation percentage. At a relaxation percentage of below 2% in
each stage, the multi-stage relaxation may lose its significance,
thus making it difficult to achieve the desired effect of
increasing the water permeation rate. This also holds true with the
case of a total relaxation percentage of below 4%. On the other
hand, a relaxation percentage exceeding 20% in each stage or a
total relaxation percentage exceeding 30% is difficult to realize
or, even if possible, can only result in a saturation or even a
decrease of the effect of increasing the water permeation rate,
while it may somewhat depend on the stretching ration in the
previous step, so that it is not desirable.
[0046] The time of relaxation treatment in each stage can be short
or long, as far as a desired relaxation percentage can be attained.
It is ordinarily on the order of 5 seconds to 1 minute, but it is
not necessary to be within the range.
[0047] A remarkable effect of the above-mentioned multi-stage
relaxation treatment is an increase in water permeation rate of the
resultant hollow-fiber porous membrane, whereas the pore size
distribution is not substantially changed and the porosity tends to
be somewhat lowered. The thickness of the porous membrane is not
substantially changed, whereas the inner diameter and the outer
diameter of the hollow-fiber membrane tend to be increased.
[0048] It is also possible to apply a heat treatment at a
relaxation percentage of 0%, i.e., a heat setting treatment, after
the multi-stage relaxation treatment of the present invention.
[0049] (Hollow-Fiber Porous Membrane of Vinylidene Fluoride
Resin)
[0050] The hollow-fiber porous membrane of vinylidene fluoride
resin according to the present invention obtained through the
above-mentioned series of steps is characterized by: (A) a ratio
Pmax (1 m)/Pm of at most 4.0 between a maximum pore size Pmax (1 m)
measured at a test length of 1 m according to the bubble point
method and an average pore size Pm of 0.05-0.20 .mu.m measured
according to the half dry method; and (B) a ratio F (L=200 mm,
v=70%)/Pm.sup.2 of at least 3000 (m/dayPm.sup.2), wherein the ratio
F (L=200 mm, v=70%)/Pm.sup.2 denotes a ratio between F (L=200 mm,
v=70%) which is a value normalized to a porosity v =70% of a water
permeation rate F (100 kPa, L=200 mm) measured at a test length
L=200 mm under the conditions of a pressure difference of 100 kPa
and a water temperature of 25.degree. C. and a square Pm.sup.2 of
an average pore size Pm. Now, a measurement method adopted in the
present invention for evaluating the property (A) will be
described.
[0051] (A) The bubble point/half point method is a method or
methods according to ASTMF316-86 and ASTME1294-86 for measuring a
maximum pore size Pmax and a pore size distribution of a porous
membrane, particularly suited for a hollow-fiber porous membrane.
More specifically, according to the bubble point method, a
compressed air is supplied into a sample hollow-fiber porous
membrane soaked in a test liquid at gradually increasing pressures
to determine an air pressure at which a first bubble is generated
in the test liquid, and a maximum pore size Pmax (.mu.m) of the
sample membrane is calculated from the air pressure. According to
the half dry method, an air pressure is determined for a sample
hollow-fiber porous membrane at an intersection of a wet flow curve
as a flow curve obtained in the state of the sample membrane being
wetted with the test liquid and a half dry curve which is defined
as a line having a slope of half inclination with respect to a dry
flow curve measured in a dry state of the sample membrane, and an
average pore size Pm (.mu.m) is calculated from the air pressure.
These values described herein are based on values measured by using
"PALM POROMTER CFP-2000AEX" made by Porous Materials, Inc., as a
measuring instrument and perfluoropolyester (trade name: "GALWICK")
as a test liquid. Hollow-fiber membranes having a test length of
ca. 10 mm are ordinarily used as samples. In the present invention,
however, in addition to the measurement of maximum pore size Pmax
and average pore size Pm at a test length L=10 mm, a measurement of
a maximum pore size Pmax (1 m) by using a hollow-fiber membrane at
a test length of 1 m is performed in order to surely capture
pinholes occurring during the production of hollow-fiber membranes
regardless of fluctuation of the production conditions.
Incidentally, Table 1 below shows 5 measured values of Pmax (1 m)
and an average thereof measured for 5 samples each taken from
hollow-fiber membranes prepared in Example 1 and Comparative
Example 1 described hereinafter in parallel with measured values of
Pmax (10 mm) for the same hollow-fiber membranes. From the results
shown in Table 1, it is believed understandable that the
inclination of pinhole occurrence can be surely evaluated based on
the values of Pmax (1 m).
TABLE-US-00001 TABLE 1 Pmax(1 m) and Pmax(10 mm) (unit: .mu.m)
Pmax(1 m) Pmax Sample No. 1 2 3 4 5 Average (10 mm) Example 1 0.266
0.364 0.305 0.272 0.267 0.295 0.259 Comp. 0.815 0.912 0.848 0.921
0.760 0.851 0.269 Example 1
[0052] The hollow-fiber porous membrane of the present invention is
characterized by an average pore size Pm of 0.05-0.20 .mu.m,
preferably 0.08-0.18 .mu.m, as measured at a test length L=10 mm; a
ratio Pmax (1 m)/Pm of at most 4.0, preferably at most 3.0, as a
ratio of the above-mentioned Pmax (1 m) measured at a test length
L=1 m to the Pm. If the average pore size Pm is below 0.05 .mu.m,
the porous membrane is caused to have a lower water permeation
rate. On the other hand, if Pm exceeds 0.20 .mu.m, the ability of
removing minute particles (such as turbidity sources and bacteria)
is lowered. The ratio Pmax (1 m)/Pm of at most 4.0 means that the
hollow-fiber porous membrane of the present invention includes very
few abnormally large pores (pinholes) in addition to a uniform pore
size distribution. The lower limit of Pmax (1 m)/Pm is not
particularly restricted, but it is difficult to realize a value
smaller than 1.5.
[0053] (B) Another characteristic feature of the hollow-fiber
porous membrane of the present invention is that it exhibits a
large water permeation rate (or water permeability) relative to a
small average pore size (that is, a good communicativeness of
pores). This characteristic is represented by a ratio F (L=200 mm,
v=70%)/Pm.sup.2 of at least 3000 (m/day.mu.m.sup.2), preferably at
least 3500 (m/day.mu.m.sup.2), wherein the ratio F (L=200 mm,
v=70%)/Pm.sup.2 denotes a ratio between F (L=200 mm, v=70%) which
is a value normalized to a porosity v=70% of a water permeation
rate F (100 kPa, L=200 mm) measured at a test length L=200 mm under
the conditions of a pressure difference of 100 kPa and a water
temperature of 25.degree. C. and a square Pm.sup.2 of an average
pore size Pm. Describing more specifically, if it is assumed that a
relationship between a water permeation rate and an (average) pore
size follows the Hagen-Poiseuille law, the water permeation rate is
proportional to a 4-th power of the (average) pore size while the
number of pores is assumed to be inversely proportional to the
square of average pore size, so that the water permeation rate is
proportional to the square of (average) pore size. The present
inventors have acquired experimental results that the square law
does not hold true at different porosities but the water permeation
rate is proportional to the porosity and have found that the square
law between the water permeation rate and the average pore size
holds true satisfactorily at a normalized constant porosity (70% in
the present invention) and, based on the relationship, the
above-mentioned F (L=200 mm, v=70%) provides a good index of water
permeability including a contribution of the capability of removing
minute particles based on the good communicativeness of pores in a
porous membrane. The property (B) of the hollow-fiber porous
membrane of the present invention is based on this knowledge. The
method for measurement of the water permeation rate per se is
described hereinafter.
[0054] Other general features of hollow-fiber porous membranes
obtained according to the present invention may include: a porosity
of 55-90%, preferably 60-85%, particularly preferably 65-80%; a
tensile strength of at least 6 MPa; an elongation at break of at
least 5%. Further, the thickness is ordinarily in the range of
5-800 .mu.m, preferably 50-600 .mu.m, particularly preferably
150-500 .mu.m. The outer diameter of the hollow fiber may suitably
be on the order of 0.3-3 mm, particularly ca. 1-3 mm.
[0055] Further, a micro-texture characteristic of the hollow-fiber
porous membrane according to the present invention obtained through
the stretching is that it comprises a crystalline oriented portion
and a crystalline non-oriented portion (random oriented portion)
recognizable by X-ray diffraction, which are understood as
corresponding to a stretched fibril portion and a non-stretched
node portion, respectively.
EXAMPLES
[0056] Hereinbelow, the present invention will be described more
specifically based on Examples and Comparative Examples. The
properties described herein including those described below are
based on measured values according to the following methods.
[0057] (Weight-Average Molecular Weight (Mw))
[0058] A GPC apparatus ("GPC-900", made by Nippon Bunko K.K.) was
used together with a column of "Shodex KD-806M" and a pre-column of
"Shodex KD-G" (respectively made by Showa Denko K.K.), and
measurement according to GPC (gel permeation chromatography) was
performed by using NMP as the solvent at a flow rate of 10 ml/min.
at a temperature of 40.degree. C. to measure polystyrene-based
molecular weights.
[0059] (Crystalline melting points Tm1, Tm2, crystal melting
enthalpy and Crystallization Temperature Tc)
[0060] A differential scanning calorimeter "DSC-7" (made by
Perkin-Elmer Corp.) was used. A sample resin of 10 mg was set in a
measurement cell, and in a nitrogen gas atmosphere, once heated
from 30.degree. C. up to 250.degree. C. at a temperature-raising
rate of 10.degree. C./min., then held at 250.degree. C. for 1 min.
and cooled from 250.degree. C. down to 30.degree. C. at a
temperature-lowering rate of 10.degree. C./min., thereby to obtain
a DSC curve. On the DSC curve, an endothermic peak temperature in
the course of heating was determined as a melting point Tm1
(.degree. C.), and a heat of absorption by the endothermic peak
giving Tm1 was measured as a crystal melting enthalpy. Further, an
exothermic peak temperature in the course of cooling was determined
as a crystallization temperature Tc(.degree. C.). Successively
thereafter, the sample resin was held at 30.degree. C. for 1 min.,
and re-heated from 30.degree. C. up to 250.degree. C. at a
temperature-raising rate of 10.degree. C./min. to obtain a DSC
curve. An endothermic peak temperature on the re-heating DSC curve
was determined as an inherent melting point Tm2 (.degree. C.)
defining the crystallinity of vinylidene fluoride resin in the
present invention.
[0061] (Porosity)
[0062] The length and also the outer diameter and inner diameter of
a sample hollow fiber porous membrane were measured to calculate an
apparent volume V (cm.sup.3) of the porous membrane, and the weight
W (g) of the porous membrane was measured to calculate a porosity
according to the following formula:
Porosity (%)=(1-W/(V.times..rho.)).times.100,
wherein .rho.: density of PVDF (=1.78 g/cm.sup.3).
[0063] (Water Permeation Rate or Water Permeability)
[0064] A sample hollow-fiber porous membrane having a test length L
(as shown in FIG. 1)=200 mm or 800 mm was immersed in ethanol for
15 min., then immersed in water to be hydrophilized, and then
subjected to a measurement of water permeation rate per day
(m.sup.3/day) at a water temperature of 25.degree. C. and a
pressure difference of 100 kPa, which was then divided by a
membrane area of the hollow-fiber porous membrane (m.sup.2) (=outer
diameter.times..pi..times.test length L) to provide a water
permeation rate. The resultant value is indicated, e.g., as F (100
kPa, L=200 mm) for a sample having a test length L=200 mm, in the
unit of m/day (=m.sup.3/m.sup.2day). It is known that as the test
length L is increased, the water permeation rate per unit membrane
area is generally lowered due to an increase in flow resistance
through the hollow fiber, so that a basic water permeation rate
F.sub.0 (m/day) was obtained by extrapolating the measured values F
at test lengths L=200 mm and 800 mm to L=0 mm.
[0065] Further, a water permeation rate F (100 kPa, L=200 mm)
measured at a test length L=200 mm was normalized at a porosity
v=70% to obtain F (L=200 mm, v=70%) according to the following
formula:
F(L=200 mm, v=70%)
[0066] =F(100 kPa, L=200 mm).times.(70 (%)/v (%)), and a ratio
thereof to the square of average pore size Pm of F (L=200 mm,
v=70%)/Pm.sup.2 was obtained as a pure water permeation flux
representing a communicativeness of pores, thereby evaluating a
water permeability while taking capability of removing minute
particles into consideration.
[0067] (Measurement of Number of General Bacteria)
[0068] The number of general bacteria, i.e., medium-temperature
aerobic bacteria, was measured by inoculating 1 mL of test water
into ca. 20 mL of standard agar medium (as a composition,
containing 2.5 g of yeast extract, 5 g of peptone, 1 g of D-glucose
and 15 g of agar in 1000 mL; pH 7.1.+-.0.1), placed in a 100
mm-dia. Petri dish, culturing it for 48 hours at 36.+-.1.degree.
C., and counting the number of colonies visible by naked eyes.
[0069] In order to evaluate the cleaning ability of sample
hollow-fiber membranes, a surface flowing water of Koisegawa-river
taken in Ishioka-city, Ibaraki-prefecture was used as a supply
water. The number of general bacteria in the supply water was
330/mL.
[0070] A sample hollow-fiber porous membrane having a test length L
(as shown in FIG. 1)=800 mm was immersed in ethanol for 15 min.,
then immersed in pure water to be hydrolyzed and, after discharging
the pure water, the pressure-resistant vessel was filled with the
supply water. After that, filtration was performed while retaining
a pressure of 50 kPa in the vessel to recover a filtered water,
which was then subjected to the above-mentioned measurement of
number of general bacteria. The tap water standard in Japan
stipulates that the number of colonies should be at most 100 in 1
mL.
Example 1
[0071] A principal polyvinylidene fluoride (PVDF) (powder) having a
weight-average molecular weight (Mw) of 4.12.times.10.sup.5 and a
crystallinity-modifier polyvinylidene fluoride (PVDF) (powder)
having Mw=9.36.times.10.sup.5 were blended in proportions of 95 wt.
% and 5 wt. %, respectively, by a Henschel mixer to obtain a PVDF
mixture having Mw=4.38.times.10.sup.5.
[0072] An adipic acid-based polyester plasticizer ("PN-150", made
by Asahi Denka Kogyo K.K.) as an aliphatic polyester and
N-methyl-pyrrolidone (NMP) as a solvent were mixed under stirring
in a ratio of 77.5 wt. %/22.5 wt. % at room temperature to obtain a
plasticizer-solvent mixture.
[0073] An equi-directional rotation and engagement-type twin-screw
extruder ("BT-30", made by Plastic Kogaku Kenkyusyo K.K.; screw
diameter: 30 mm, L/D=48) was used, and the PVDF mixture was
supplied from a powder supply port at a position of 80 mm from the
upstream end of the cylinder and the plasticizer-solvent mixture
heated to 160.degree. C. was supplied from a liquid supply port at
a position of 480 mm from the upstream end of the cylinder at a
ratio of PVDF mixture/plasticizer-solvent mixture=35.7/64.3 (wt.
%), followed by kneading at a barrel temperature of 220.degree. C.
to extrude the melt-kneaded product through a nozzle having an
annular slit of 7 mm in outer diameter and 5 mm in inner diameter
into a hollow fiber-form extrudate at a rate of 7.6 g/min. In this
instance, air was injected into a hollow part of the fiber at a
rate of 4.2 mL/min. through an air supply port provided at a center
of the nozzle.
[0074] The extruded mixture in a molten state was introduced into a
cooling bath of water maintained at 40.degree. C. and having a
surface 280 mm distant from the nozzle (i.e., an air gap of 280 mm)
to be cooled and solidified (at a residence time in the cooling
bath of ca. 6 sec.), pulled up at a take-up speed of 5 m/min. and
wound up about a reel of ca. 1 m in diameter to obtain a first
intermediate form.
[0075] Then, the first intermediate form was immersed under
vibration in dichloromethane at room temperature for 30 min.,
followed by immersion in fresh dichloromethane again under the same
conditions to extract the plasticizer and solvent and further by 1
hour of heating in an oven at 120.degree. C. for removal of the
dichloromethane and heat treatment, thereby to obtain a second
intermediate form.
[0076] Then, the second intermediate form was longitudinally
stretched at a ratio of 2.2 times by passing it by a first roller
at a speed of 8.0 m/min., through a water bath at 60.degree. C. and
by a second roller at a speed of 17.6 m/min. Then, the intermediate
form was caused to pass through a bath of warm water controlled at
90.degree. C. and by a third roller at a lowered speed of 15.1
m/min. to effect a 14%-relaxation treatment in the water bath. The
form was further caused to pass through a dry heating bath (of 2.0
m in length) controlled at a spatial temperature of 140.degree. C.
and by a fourth roller at a lowered speed of 14.5 m/min. to effect
a 4%-relaxation treatment in the dry heating bath and was taken up
to provide a polyvinylidene fluoride-based hollow-fiber porous
membrane (a third form) according to the process of the present
invention.
[0077] The resultant polyvinylidene fluoride-based hollow-fiber
porous membrane exhibited an outer diameter of 1.275 mm, an inner
diameter of 0.836 mm, a membrane thickness of 0.219 mm, a porosity
of 69.9%, pure water permeabilities F (L, 100 kPa) at a pressure
difference of 100 kPa including F (L=200 mm, 100 kPa)=86.7 m/day at
a test length L=200 mm, F (L=200 mm, v=70%)=86.8 m/day as a value
normalized at a porosity of 70%, F (L=800 mm, 100 kPa)=73.5 m/day
and a basic water permeability F.sub.0(L=0 mm, 100 kPa)=91.1 m/day,
an average pore size Pm=0.135 .mu.m, a maximum pore size Pmax (10
mm)=0.259 .mu.m at a test length of 10 mm, Pmax (10 mm)/Pm=1.9, and
a ratio F (L=200 mm, v=70%)/Pm.sup.2=4745. Further, the number of
general bacteria in the filtered water was 0/mL.
[0078] The production conditions and physical properties of the
thus-obtained polyvinylidene fluoride-based hollow-fiber porous
membrane are inclusively shown in Table 2 appearing hereinafter
together with the results of the following Examples and Comparative
Examples.
Example 2
[0079] A hollow-fiber porous membrane was prepared in the same
manner as in Example 1 except for changing the ratio of the
components in the plasticizer-solvent mixture to 82.5 wt. %/17.5
wt. %, the composition extrusion rate to 16.7 g/min., the air
supply rate through the air supply port at the nozzle center to 9.5
mL/min., the water-bath temperature to 40.degree. C., the take-up
speed to 11.0 m/min., the stretching ratio to 1.85 times, the
first-stage relaxation to 8% by wet heat at 90.degree. C., and the
second-stage relaxation to 4% by dry heat at 140.degree. C.,
respectively.
Example 3
[0080] A hollow-fiber porous membrane was prepared in the same
manner as in Example 2 except for changing the composition
extrusion rate to 15.0 g/min., the air supply rate through the air
supply port at the nozzle center to 6.5 mL/min., the take-up speed
to 10.0 m/min., the heat-treatment temperature to 80.degree. C.,
the first-stage relaxation to 8% by dry heat at 140.degree. C., and
the second-stage relaxation to 2% by dry heat at 140.degree. C.,
respectively.
Comparative Example 1
[0081] A hollow-fiber porous membrane was prepared in the same
manner as in Example 1 except for changing the air supply rate
through the air supply port at the nozzle center to 4.5 mL/min.,
the stretching ratio to 1.9 times, the first-stage relaxation to 5%
in the medium of dichloromethane, and the second-stage relaxation
to 5% by dry heat at 140.degree. C., respectively.
Comparative Example 2
[0082] A hollow-fiber porous membrane was prepared in the same
manner as in Example 3 except for changing the air supply rate
through the air supply port at the nozzle center to 7.0 mL/min.,
the heat treatment temperature to 120.degree. C., the first-stage
relaxation to 5% in the medium of dichloromethane, and the
second-stage relaxation to 5% by dry heat at 140.degree. C.,
respectively.
Comparative Example 3
[0083] A hollow-fiber porous membrane was prepared in the same
manner as in Example 3 except for changing the heat-treatment
temperature to 120.degree. C., omitting the relaxation treatments
and performing a heat-setting by dry heat at 140.degree. C. instead
thereof.
Comparative Example 4
[0084] A hollow-fiber porous membrane was prepared in the same
manner as in Example 3 except for omitting the relaxation
treatments and performing two stages of heat setting by dry heat at
140.degree. C. instead thereof.
Comparative Example 5
[0085] A hollow-fiber porous membrane was prepared in the same
manner as in Example 3 except for changing the first-stage
relaxation to 10% by dry heat at 140.degree. C., omitting the
second-stage relaxation and performing heat-setting by dry heat at
140.degree. C.
[0086] The production conditions of the above-described Examples
and Comparative Examples, and the physical properties of the
resultant hollow-fiber porous membranes are inclusively shown in
the following Table 2.
TABLE-US-00002 TABLE 2 Example Comp. 1 2 3 1 Starting PVDF mixture
Mw (.times.10.sup.5) 4.38 4.38 4.38 4.38 material Plasticizer
solvent mixture 72.5/27.5 82.5/17.5 82.5/17.5 72.5/27.5 composition
Plasticizer/solvent (wt. ratio) PVDF/plasticizer solvent supply
ratio (wt. ratio) 35.7/64.3 35.7/64.3 35.7/64.3 35.7/64.3
Production Strting material feed rate (g/min.) 7.6 16.7 15.0 7.6
conditions Air supply rate (mL/min.) 4.2 9.5 6.5 4.5 Water bath
temp. (.degree. C.) 50 40 40 50 Take-up speed (m/min) 5.0 11.0 10.0
5.0 Extraction bath CH.sub.2Cl.sub.2 CH.sub.2Cl.sub.2
CH.sub.2Cl.sub.2 CH.sub.2Cl.sub.2 Dry heat temp. (.degree. C.) 120
120 80 120 Stretch temp. (.degree. C.) 60 60 60 60 Stretch ratio
(--) 2.2 1.85 1.85 1.9 Relaxation 1st-stage: conditions Wet heat
Wet heat Dry heat Wetting* 90.degree. C. 90.degree. C. 140.degree.
C. relaxation % 14% 8% 8% 5% 2nd-stage: conditions Dry heat Dry
heat Dry heat Dry heat 140.degree. C. 140.degree. C. 140.degree. C.
140.degree. C. relaxation % 4% 4% 2% 5% Physical Outer diameter
(mm) 1.275 1.368 1.247 1.305 properties Inner diameter (mm) 0.836
0.878 0.758 0.841 Thickness (mm) 0.219 0.245 0.244 0.232 Porosity v
(%) 69.9 71.1 69.7 70.4 Average pore size Pm (.mu.m) 0.135 0.115
0.107 0.126 Pmax(10 mm) (.mu.m) 0.259 0.207 0.172 0.269 Pmax(1 mm)
(.mu.m) 0.295 0.266 0.208 0.851 Pmax(10 mm)/Pm 1.91 1.80 1.61 2.14
Pmax(1 m)/Pm 2.18 2.31 1.95 6.75 Water permeability F (100 kPa, L =
200 mm) (m/day) 86.7 47.2 37.3 85.7 Water permeability F (100 kPa,
L = 800 mm) (m/day) 73.5 43.2 31.9 72.5 Basic water permability Fo
(100 kPa, L = 0 mm) (m/day) 91.1 48.5 39.1 90.1 Normalized water
permeability F (L = 200 mm, v = 70%) (m/day) 86.8 46.5 37.5 85.2 F
(L = 200 mm, v = 70%)/Pm.sup.2 (m/day .mu.m.sup.2) 4745 3514 3286
5408 Number of general bacteria (--/mL) 0 0 0 5 Example Comp. Comp.
Comp. Comp. 2 3 4 5 Starting PVDF mixture Mw (.times.10.sup.5) 4.38
4.38 4.38 4.38 material Plasticizer solvent mixture 82.5/17.5
82.5/17.5 82.5/17.5 82.5/17.5 composition Plasticizer/solvent (wt.
ratio) PVDF/plasticizer solvent supply ratio (wt. ratio) 35.7/64.3
35.7/64.3 35.7/64.3 35.7/64.3 Production Strting material feed rate
(g/min.) 15.0 15.0 15.0 15.0 conditions Air supply rate (mL/min.)
7.0 6.5 6.5 6.5 Water bath temp. (.degree. C.) 40 40 40 40 Take-up
speed (m/min) 10.0 10.0 10.0 10 Extraction bath CH.sub.2Cl.sub.2
CH.sub.2Cl.sub.2 CH.sub.2Cl.sub.2 CH.sub.2Cl.sub.2 Dry heat temp.
(.degree. C.) 120 120 80 80 Stretch temp. (.degree. C.) 60 60 60 60
Stretch ratio (--) 1.85 1.85 1.85 1.85 Relaxation 1st-stage:
conditions Wetting* none Dry heat Dry heat 140.degree. C.
140.degree. C. relaxation % 5% 0% 0% 2nd-stage: conditions Dry heat
Dry heat Dry heat Dry heat 140.degree. C. 140.degree. C.
140.degree. C. 140.degree. C. relaxation % 5% 0% 0% 0% Physical
Outer diameter (mm) 1.297 1.208 1.187 1.242 properties Inner
diameter (mm) 0.807 0.738 0.723 0.758 Thickness (mm) 0.245 0.235
0.232 0.242 Porosity v (%) 70.7 70.5 68.8 69.1 Average pore size Pm
(.mu.m) 0.105 0.105 0.096 0.106 Pmax(10 mm) (.mu.m) 0.203 0.195
0.162 0.186 Pmax(1 mm) (.mu.m) 0.753 0.251 0.208 0.251 Pmax(10
mm)/Pm 1.93 1.86 1.69 1.75 Pmax(1 m)/Pm 7.17 2.39 2.17 2.36 Water
permeability F (100 kPa, L = 200 mm) (m/day) 44 30.8 26.6 33.1
Water permeability F (100 kPa, L = 800 mm) (m/day) 39.5 27.3 23.0
28.8 Basic water permability Fo (100 kPa, L = 0 mm) (m/day) 45.5
32.0 27.8 34.5 Normalized water permeability F (L = 200 mm, v =
70%) (m/day) 43.6 30.6 27.1 33.5 F (L = 200 mm, v = 70%)/Pm.sup.2
(m/day .mu.m.sup.2) 3951 2774 2936 2974 Number of general bacteria
(--/mL) 3 0 0 0 *Wetting liquid: dichloromethane
(.dbd.CH.sub.2Cl.sub.2), 25.degree. C.
[0087] The results in the above Table 2 show that the hollow-fiber
porous membrane of the present invention obtained by subjecting a
stretched hollow-fiber porous membrane of vinylidene fluoride resin
to two stages of relaxation treatment in a non-wetting environment
(wet heat with warm water and/or dry heated air) exhibited
performances, which are very suitable for water treatment,
including both (A) a stably narrow pore size distribution
represented by a Pmax (1 m)/Pm of at most 4.0 and (B) a large water
permeation rate in combination with good minute particle-blocking
performance as represented by F (L=200 mm, v=70%)/Pm.sup.2 of at
least 3000. In contrast thereto, the products of Comparative
Examples 1 and 2 wherein the first-stage relaxation treatment was
performed in a liquid (dichloromethane) wetting vinylidene fluoride
resin, exhibited a large effect of increasing the water permeation
rate but resulted in Pmax (1 m)/Pm>4.0 which suggests the
formation of pinholes leading to passing-by of general bacteria
during water filtration. On the other hand, the products of
Comparative Examples 3 and 4 obtained without substantial
relaxation treatment and the product of Comparative Example 5
obtained through a single-stage of relaxation treatment in a
non-wetting environment, failed to exhibit (B) the effect of
increasing the water permeation rate at a desired level.
* * * * *